The present invention relates generally to stepping motors and more particularly to circuits for controlling the energization of stepping motors.
In stepping motors, it is common to energize the field windings of a motor in a sequential step-by-step manner which will cause the armature of the motor to rotate in a corresponding step-by-step manner. The degree of mechanical rotation of the rotor is dependent on the number of field windings, the location of the field windings with respect to the rotor, and the manner in which the field windings are energized. The prior art is replete with stepping motor control circuits which rotate the motor armature in full steps. While such circuits have been fairly effective in advancing the motor armature at high speeds, at low speeds the motor armatures have been subject to resonance problems which cause the armature to resonate or oscillate or exhibit radical variances in velocity.
A number of circuits designed to drive a stepping motor in half steps are known in the art as in the fact that a motor driven in half steps does not resonate as severely as a motor driven in full steps. To help explain this phenomenon, consider that stepping motor low frequency resonance closely approximates a linear underdamped spring-mass system. In such a system halving the input command halves the output displacement. However, motor torque is not linear with respect to displacement. This is because damping is a combination of windage and friction, and while the windage in a stepping motor is linear, the friction is not. Thus, when operating a stepping motor in a continuous fashion, if the minimum potential energy due to the motor torque over a step is less than the energy dissipated by the friction torque, the motor will behave in an overdamped manner. The motor torque is proportional to the sine of the displacement, and as a result the minimum potential energy for a step decreases with the step size. The potential energy, E, is equal to: ##EQU1## where K is the motor holding torque, S is the step size and where a full step S equals .pi./2, and B is the angular displacement remaining at the end of a step. The minimum potential energy occurs when B = 0.
U.S. Pat. No. 3,445,741 discloses a technique for producing fractional steps to control a stepping motor. This system requires that for producing N sub-steps in a four phase stepping motor, 2N output transistors and associated logic circuitry must be utilized. Thus, in order to produce very fine steps this circuit becomes quite complex.